In conventional vehicles, i.e., vehicles that are neither hybrid vehicles nor electric vehicles, accessories such as hydraulic pumps, water pumps, vacuum pumps, and compressors for air brakes or air conditioning systems are powered directly from the internal combustion engine. The power required by these accessories may be steady over extended periods of time, as in the case of a water pump that is used to help cool the engine, or it may be required in short bursts when needed by the accessory, as in the case of hydraulic accessories or air brakes. Because such accessories have relatively high power requirements, the most economical and efficient source of power for these accessories is the internal combustion engine.
In hybrid vehicles that include an internal combustion engine, either in series or in parallel with the electric motor, these same accessories are often powered from the internal combustion engine in much the same way as in a conventional vehicle. Alternatively, in some hybrid vehicles and in most electric vehicles, the traditional accessories are replaced with a wholly electric version of the accessory to perform the same function, such as an electric air conditioning system or an electrically powered compressed air brake system.
In implementing either of the above solutions to power vehicle accessories, however, inefficiencies are introduced into a vehicle. These inefficiencies may affect the cost of the vehicle, the performance of the vehicle, or both. For example, if a hybrid or electric vehicle includes electric versions of one or more accessories, the inefficiency introduced in the hybrid vehicle is in the overall cost of the vehicle. The increased cost arises because the electric version of an accessory must initially be developed and tested. Additionally, the cost of the electric version of the accessory will typically remain high over a period of one to several years because vehicles which use the electric version of the accessory do not have the high production quantities of conventional vehicles.
Conversely, if a hybrid vehicle implements a vehicle accessory in the traditional manner, i.e., powered directly from an internal combustion engine, the vehicle will experience inefficiencies in fuel economy. The inefficiencies arise when the internal combustion engine provides power directly to the accessories and the electric motor and/or the batteries do not require power. During these periods, the engine necessarily consumes fuel to provide the power required by the accessories. Any excess power generated by the engine, however, is lost because it cannot be otherwise used or stored by the vehicle.
Series and parallel hybrid-hydraulic drive systems for heavy-duty vehicles have been developed by Eaton Corporation, Dana Corporation, Permo Drive, and others, but none of these developments have included separate hydraulically driven accessories.